Polymeric materials are useful as viscosity enhancers when dissolved in the appropriate solvent system. The principle reason for this behavior is due primarily to the large volume which a single macromolecular chain can occupy within the solvent. An increase in the size of the chain produces a concomitant enhancement in the solution viscosity. However, when the polymer chain is placed in a shear field, segmental orientation takes place in the direction of the shearing force. The viscosity of the fluid dramatically drops due to this orientation phenomena. This is a typical behavior of most solutions containing dissolved polymeric materials. However, if the polymer molecule has a high molecular weight with a relatively flexible backbone and the solvent viscosity is sufficiently high, different behavior can be anticipated. It has been shown by several groups that, with increasing shear rates, the viscosity should show a decrease, followed by a minimum value and a small subsequent increase in cases where both solvent viscosity and polymer molecular weight are very high. This latter effect gives rise to a slight dilatant behavior. However, the above-mentioned conditions required for the appearance of a slight shear thickening behavior in these polymeric solution systems are not applicable for many technologically interesting fluids. In most of the common synthetic polymers, it is difficult from a synthetic viewpoint to obtain sufficiently high molecular weight and, in addition, most solvents (for example, jet fuels) have rather low viscosities.
U.S. Pat. No. 4,002,436 discloses hydrocarbon jet fuel compositions of addition polymers with associative polar bonds. However "the polymers used in [that] invention will generally be of the free radical addition type since these are the simplest to make in the presence of polar groups" (column 4, lines 16-18). Alkene hydrocarbon or alpha olefin polymers usually require post polymerization reaction to introduce the polar groups and are thus generally excluded from this invention (column 4, lines 58-68, and column 5, lines 1-3). Moreover, compositions of U.S. Pat. No. 4,002,436 make no claims to showing dilatant or shear thickening rheology (i.e. instantaneous increase in viscosity, with increasing shear rate).
This invention discloses the novel and unexpected result that polymers containing low levels of carboxylic acid side groups are capable of enhancing the viscosity of hydrocarbon solutions under relatively broad shear conditions. With these unique polymeric materials, dilatant behavior occurs in hydrocarbon fluids which are of broad technological utility and useful in antimisting applications, preferably for jet fuel. It is further observed that under the identical experimental conditions, the viscosity of related copolymer solutions containing no carboxylic acid side groups show the normal shear thinning behavior.
Polymers with very high molecular weight can be used to modify a solvent for antimisting behavior. In this invention it is disclosed that an alternative to ultra high molecular weight additives are lower molecular weight polymers which are capable of associating in solution, thereby building a network of a very high molecular weight. A way for achieving such networks is the association of polymers containing a low level of carboxylic acid side groups.
In order to avoid phase separation of the associating polymer in solution, the acid density along the polymer backbones should be relatively low. The resulting solution of such a polymer is then significantly more viscous than solutions containing related polymers which do not associate. Upon addition of a strongly polar agent such as an alcohol the associations can be disturbed and the viscosity reduced.
It was found that for a given range of the various parameters that may be varied in a carboxylic acid containing polymer solution, an unexpected shear thickening (dilatant) behavior may be obtained. These parameters include:
Backbone nature of the polymer (or copolymer). PA1 The polar group densities along the polymer backbones. PA1 The molecular weight of the polymer. PA1 The solvent (and cosolvent, if any). PA1 The concentration of polymer in solution.
As explained above, most solutions of high molecular weight polymers are expected to exhibit a shear thinning behavior. Polymers containing carboxylic acid side groups under narrow conditions seem on the other hand to possess an ability to establish even larger networks or act as if networks are larger under high shear rates resulting in shear thickening.
Shear thickening behavior can be useful in affecting antimisting characteristics. Such a solution can behave as a fairly low viscosity fluid at low shear rates. However, the viscosity begins to rise as the shear rate is progressively increased. Accordingly, the solution can more effectively resist breakup into a mist of minute droplets. This is a very desirable attribute in a variety of fluids of technological interest and specifically jet fuels. Another desirable attribute is to be able to reverse (or erase) the above-mentioned antimisting behavior and render it atomizable. With regard to polymers of the instant invention, this is readily achieved through addition of a soluble component capable of weakening or totally disrupting the associations which hold the network together. Such a component should be highly polar, soluble in the solution containing the dissolved associating polymer and capable of efficiently migrating (and disrupting) the associations. Alcohols and amines and low molecular weight carboxylic acids are only a few of many possible examples.
U.S. Pat. No. 3,679,382 teaches the thickening of aliphatic hydrocarbons with synthetic organic polymers of alkylstyrene or lauryl acrylate and olefinically unsaturated emulsion copolymerizable acids, amides, hydroxyacrylic esters, sulfonic acids, etc. It is emphasized in this patent (column 3, lines 69-75) that it is critical that in the preparation of such polymers, no surface active agent, catalyst or other additive be employed which introduces a metallic ion into the system. Therefore, it is preferred to employ ammonium or amine salts.
The instant invention contains no alkylstyrenes or lauryl acrylates and the instant olefinically unsaturated acids and esters are not emulsion copolymerizable. The instant acids and esters have an alkyl spacer group separating the acid or ester from the olefinic double bond thus rendering them nonpolymerizable by free radical emulsion polymerization. The instant copolymers are prepared by Ziegler-Natta polymerization which do indeed use metallic catalysts. Such metallic species are precluded from U.S. Pat. No. 3,679,382.
Finally, the novel polymeric systems described here are readily soluble in aliphatic as well as aromatic hydrocarbons making them useful as additives to jet fuels and other distillates of crude oil.